1. Field of the Invention
The present invention relates to an method and apparatus for measuring a propagation speed of a ultrasonic wave through various mediums such as liquids (for example, liquid ethanol) or solids. Specifically the invention relates to a method and apparatus for determining a ultrasonic propagation speed through an object, by observing a period of ultrasonic echo reflected from one end of the measured object, while applying an ultrasonic wave on the other end of the measured object.
2. Description of the Background Art
As is well known, there have been disclosed two conventional methods for measuring a propagation speed of ultrasonic wave transmission through various media. One such conventional ultrasonic speed measuring method is a "Pulse Echo Superposition Method", (herein abbreviated to "PES Method"), proposed by H. J. McSkimin. The PES Method has been detailed in "THE JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA", Vol. 33, No. 1, 1961, pp12-16. The other conventional ultrasonic speed measuring method is a "Pulse Echo Overlap Method", (herein abbreviated to "PEO Method"), proposed by E. P. Papadakis et al. The PEO Method has been detailed in "THE JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA", Vol. 42, No. 5, 1967, pp1045-1051.
In both the aforementioned conventional ultrasonic speed measuring methods, the propagation speed through a measured sample, having a predetermined dimension, can be obtained by deriving a calculated value 2X/t.sub.r from a predetermined length X of the sample and a period t.sub.r of an ultrasonic echo reflected from one end of the sample, while applying an ultrasonic wave on the other end of the sample.
As shown in FIGS. 1A-1D, according to the PES Method, a plurality of exciting pulses for exciting ultrasonic waves are generated at a predetermined period of time t.sub.e, as seen in FIG. 1A. As is generally known, the exciting pulses are formed with at least two radio-frequency pulses (herein abbreviated to "RF pulses"), each RF pulse having a predetermined pulse length t.sub.p and a predetermined frequency f.sub.rf, i.e., a predetermined period t.sub.rf. FIG. 1A shows an exemplified state wherein two RF pulses, namely a first RF pulse and a second RF pulse, are generated for exciting an output ultrasonic wave. In response to the exciting RF pulses, the output ultrasonic wave having the predetermined period of time t.sub.e is applied on one end of a measured sample through, for example, a piezoelectric transducer. Thereafter, the transducer receives a supersonic echo reflected from the other end of the sample and transduces sound pressure of the ultrasonic echo to an electric signal which will be referred to as an "ultrasonic echo signal". The ultrasonic echo signal can be observed on the screen of an oscilloscope. Note that, in the PES Method, a superposed echo, generated by superposition of ultrasonic echos occurring due to output ultrasonic waves excited by RF pulses is actually received by the transducer, since the exciting RF pulses are generated in close proximity to each other. For example, FIGS. 1B and 1C show an unsuperposed ultrasonic echo signal occurring via the first RF pulse and an unsuperposed ultrasonic echo signal occurring via the second RF pulse, respectively. On the other hand, FIG. 1D shows a superposed ultrasonic echo signal waveform actually observed on the screen of an oscilloscope, obtained by superposing the two echo signals shown in FIGS. 1B and 1C, under a condition wherein the predetermined period t.sub.e of the output ultrasonic wave (the exciting RF pulse) is consistent with a period of time t.sub.r of the ultrasonic echo or the ultrasonic echo signal. As seen in FIG. 1D, if the period of time t.sub.e of the exciting RF pulse is consistent with the period of time t.sub.r of the echo signal, the superposed ultrasonic echo signal provides the greatest amplitude by superposing the two echo signals. Therefore, a period of time t.sub.r of a ultrasonic echo signal can be obtained by monitoring a particular period of time t.sub.e of the RF pulse when the amplitude of the superposed ultrasonic echo signal, which is observed on an oscilloscope, becomes a maximum value. An ultrasonic speed of an ultrasonic wave traversing the measured sample can be calculated on the basis of a period t.sub.r of an ultrasonic echo signal, measured in accordance with the procedure as previously described.
However, as appreciated from FIGS. 1A-1D, in the aforementioned PES Method, the amplitude of the superposed ultrasonic echo signal may become greater, even if a period of time t.sub.e of the RF pulse is equal to a period of time t.sub.r .+-.k.t.sub.rf, wherein k is an integer, as well as a period of time t.sub.r of the echo signal. Therefore, to determine whether the period of time t.sub.e of the RF pulse is precisely consistent with the period of time t.sub.r of the ultrasonic echo signal, the condition of k=0 must be monitored and satisfied through visual observation on the screen of an oscilloscope. Although in FIGS. 1A-1D, the number of waves of the RF pulse or the ultrasonic echo signal is illustrated as being extremely small for the purpose of simplification of the disclosure of the present invention, the number of waves of the actually utilized RF pulse is relatively large, substantially 30 with regard to one RF pulse, as the actual RF pulse has a predetermined pulse length t.sub.p of 2 .mu.s, a period t.sub. rf of 0.067 .mu.s, a frequency f.sub.rf of 15 MHz, for example. Therefore, the determination of the condition of k=0 through visual observation on the screen is practically impossible.
For this reason, when an ultrasonic propagation speed through various media of a predetermined size is actually measured in accordance with the PES Method, the operator must perform a particular procedure wherein the period t.sub.rf of the RF pulse is slightly varied so as to precisely determine the condition of t.sub.e =t.sub.r, i.e., k=0. According to the particular procedure, the operator may judge that the condition of k=0 is satisfied if the amplitude time period of a superposed echo signal does not vary when the period t.sub.rf of the RF pulse is varied under a condition wherein the period t.sub.e of the exciting RF pulse is adjusted such that the amplitude of the superposed echo signal becomes greatest. The equation t.sub.e =t.sub.r is satisfied regardless of various values of the period of t.sub.rf of the RF pulse when the condition of k=0 is satisfied, as appreciated from the equation t.sub.e =t.sub.r .+-.k.t.sub.rf, wherein k is an integer. Such a procedure, including selecting adjustment of the period t.sub.rf as well as the period t.sub.e, is troublesome and requires a high degree of skill. Both adjustments of the periods t.sub.e and t.sub.rf require a relatively long measuring time.
As shown in FIGS. 6A-6C, according to the PEO Method, an exciting pulse for exciting an ultrasonic wave is cyclically generated at a predetermined period of time t.sub.i, as seen in FIG. 6B. Simultaneously, a X-axis trigger pulse for an oscilloscope is generated at a predetermined period of time t.sub.o (wherein t.sub.o =t.sub.i /m and m=10, 100, or 1000, for example), as seen in FIG. 6C. In response to the cyclically generated exciting pulses, an output ultrasonic wave with the predetermined period of time t.sub.i is applied on one end of a measured sample through a piezoelectric transducer and thereafter the transducer receives an ultrasonic echo reflected from the other end of the sample and generates an ultrasonic echo signal. The ultrasonic echo signal is input on the Y-axis of the oscilloscope, while the X-axis trigger pulse is input on the X-axis of the oscilloscope. When the period of time t.sub.o of the trigger pulse is consistent with a period of time t.sub.r of the ultrasonic echo signal, the cyclically input echo signals are overlapped on each other in a same phase on the screen of the oscilloscope, as is generally known. In this manner, a period of time t.sub.r of the echo signal can be obtained by monitoring the period t.sub.o of the trigger pulse when the echo signals lie in a same phase while varying the period t.sub.o for the trigger pulse. An ultrasonic speed of an ultrasonic wave traversing a measured sample can be calculated on the basis of a period t.sub.r of an ultrasonic echo signal, measured in accordance with the procedure as previously noted.
However, in the PEO Method, it is difficult to determine whether or not the input ultrasonic echo signals lie in a same phase through visual observation on the screen of the oscilloscope, because the number of waves of the actually received echo signal is relatively large although the wave number of the echo signal is relatively small for the purpose of simplification of the disclosure of the present invention. Therefore, in the PEO Method, a quick measurement of an ultrasonic speed is difficult. Moreover, it will be appreciated that determination of phase matching of ultrasonic echo signals according to the PEO Method is difficult to automatize, as compared with monitoring a maximum amplitude of a superposed ultrasonic echo signal according to the PES Method.